1. Field of the Invention
This invention relates to a super high tensile steel wire used for rubber product reinforcement, a steel cord using this steel wire and a radial tire using this steel cord.
2. Prior Art
As reinforcing materials for rubber products (elastomer products) such as automobile tires, conveyor belts and high-pressure hoses, steel wire and steel cord made by twisting together a plurality of steel wires have been used. Steel wire is also called steel filament, and steel cord is also called steel cable.
These kinds of reinforcing material, due to the conditions in which they are used and the purposes for which they are used, are required to have excellent characteristics of strength, toughness and fatigue resistance. Also, recently, with respect to rubber products, there has been a strong demand for reductions in cost, increases in ease of handling and weight reductions. In particular, weight reduction is considered important in automobile tires from such points of view as that of reducing fuel consumption, and for this reason increases in the strength of steel wire are becoming still more necessary.
However, when such strength increases are simply sought by just raising the degree of working in the wire drawing process, the toughness of the steel wire deteriorates and as a result the drawing and twisting processes become problematic and it is not possible to obtain a steel wire with the necessary characteristics. Also, the problem arises that, when a plurality of steel wires are twisted together to make a steel cord, the decrease in strength caused by this twisting is great and consequently the reason for having increased the strength of the steel wire is lost.
Conventionally, steel wire has usually been manufactured by drawing to an intermediate diameter of a predetermined value and then sequentially heat-treating, plating and drawing a carbon steel wire rod material whose carbon content is about 0.70 to 0.75 wt %. The tensile strength Y of the steel wire in this case, generally, as shown in FIG. 1, in relation to the wire diameter d, has been in the range Y.gtoreq.-1960d+3283 (N/mm.sup.2). To respond to subsequent demands for higher strength, using carbon steel wire rod material whose carbon content is about 0.80 to 0.89 wt %, the practical use of high tensile steel wire having a tensile strength Y.gtoreq.-1960d+3577 (N/mm.sup.2) has been realized.
However, to respond to recent demands of the kind mentioned above a super high strength of the level Y.gtoreq.-1960d+3920 (N/mm.sup.2) is necessary, and the present situation is that with the carbon content range mentioned above, the practical use of steel wire of this strength has not been realized due to problems of manufacturing and toughness deterioration.
More specifically, the manufacture in itself of a steel wire of a super high strength in the range Y.gtoreq.-1960d+3920 is probably possible. That is, for example by using a high carbon steel wire rod material having a high carbon content of over 1.0 wt %, making the degree of drawing large also and thereby raising strength by work hardening, it would probably be possible to manufacture such a wire.
However, in practice there are many problems. First, there are manufacturing problems. That is, firstly, carbon steel wire rod material having a high carbon content is high in cost and the heat-treatment in manufacturing is also difficult. Secondly, also in the drawing process for making a steel wire, when a high strength (high hardness) material is drawn using a die the drawing force is high and wear of the die is severe and also it often tends to happen that drawing becomes impossible and the steel wire breaks. As a result, the desired steel wire cannot be obtained practically. Overcoming this problem is difficult.
The next problem is physical characteristics. That is, the characteristics of a steel wire are not satisfactory when it just has super high strength, and it must have good toughness at the same time. This is because steel wires, unlike sheets and bars, are placed under peculiar conditions such that not only do bending and pulling forces simply act on them but also when made into a steel cord they are twisted and subjected to complex and various forces such as tensile, compression and shearing forces while embedded in a rubber matrix. However, there have not been effective toughness parameters which take account of these kinds of conditions of use.
That is, conventionally, to determine the toughness of a steel wire, the steel wire has been twisted about its central axis and the number of turns (twist value) until the steel wire breaks has been taken as a measure of its toughness. However, with this kind of twist value it has not been possible to set a strict threshold between good and poor toughness, and it has not been reliable as a measure. The reason for this is that in practice it has been found many times that even among steel wires having the same twist value, in subsequent twisting performance and fatigue resistance and so on there are good ones and poor ones.
For belt reinforcement of radial tires for vehicles, for example tires for passenger cars, in consideration of maneuvering stability, steel cords of 1.times.4 structure or 1.times.5 structure made by twisting together four or five steel wires have been widely used.
However, steel cords of 1.times.4 structure or 1.times.5 structure generally have the kinds of cross-sectional shape shown in FIG. 9-A and FIG. 9-B, and have almost no gaps between the steel wires. Consequently, in a vulcanizing process after tire molding, rubber does not readily penetrate as far as the inside of the steel cord and spaces not filled with rubber exist in the length direction in the center of the steel cord.
As a result, when the tire is cut by stones and pieces of metal during travel, water penetrates through these cuts, water reaches the steel cord and also water passes into the space in the center of the steel cord. This causes rust to propagate in the length direction of the steel cord. As a result, the adhesion between the steel cord and the rubber matrix around it adhered together by vulcanization is destroyed, so-called separation occurs and consequently the life of the tire decreases markedly and also the problem arises that the functioning of the tire as a composite deteriorates greatly.
Furthermore, as described above, to reduce automobile fuel consumption, reductions in tire weight are being strongly demanded, and at the same time the demand for cost reductions has also been stronger. As measures for lowering tire weight, making steel cords stronger and reducing the amount of cord used per unit and lowering the diameter of the steel cord and reducing the amount of rubber covering this are effective. However, so far, as mentioned above, these measures have only made it possible to realize the practical use of steel wire of high strength in the range of Y.gtoreq.-1960d+3577 (N/mm.sup.2), and this has still been insufficient.
To reduce cost, reducing the number of steel wires constituting the steel cord to three is effective. However, when the steel cord structure is simply made a 1.times.3 structure, because it has the kind of cross-sectional shape shown in FIG. 9-C, problems of rubber penetration again arise. Moreover, to maintain the strength required of the steel cord it is necessary to make the steel wire diameter large, but when the steel wire diameter is made large its fatigue resistance with respect to bending decreases greatly due to the effect of filament diameter. Therefore, the steel wire constituting the steel cord must have extremely high strength and also good toughness, and hitherto there has not been a steel wire having these characteristics and it has not been possible to respond to the demands mentioned above.